A motion transmission system in a bicycle comprises a chain extending between toothed wheels associated with the axle of the pedal cranks and with the hub of the rear wheel. When there is more than one toothed wheel at at least one of the axle of the pedal cranks and the hub of the rear wheel, and the motion transmission system is therefore provided with a gearshift, a front derailleur and/or a rear derailleur are provided for. In the case of an electronically servo-assisted gearshift, each derailleur comprises a chain guide element, also known as cage, movable to move the chain among the toothed wheels in order to change the gear ratio, and an electromechanical actuator to move the chain guide element. The actuator in turn typically comprises a motor, typically an electric motor, coupled with the chain guide element through a linkage such as an articulated parallelogram, a rack system or a worm screw system, as well as a sensor or transducer of the position, speed, acceleration and/or direction of rotation, of the rotor or of any moving part downstream of the rotor, down to the chain guide element itself. It is worthwhile noting that slightly different terminology from that used in this context is also in use.
Control electronics changes the gear ratio automatically, for example based on one or more detected variables, such as the travel speed, the cadence of rotation of the pedal cranks, the torque applied to the pedal cranks, the slope of the travel terrain, the heart rate of the cyclist and similar, and/or the gear ratio is changed based on commands manually input by the cyclist through suitable control members, for example levers and/or buttons.
A device or unit for controlling the front derailleur and a device or unit for controlling the rear derailleur—or just one of the two in the case of simpler gearshifts—are so mounted as to be easy for the cyclist to manoeuvre, normally on the handlebars, close to the handgrips thereof where the brake lever for controlling the front and rear wheel brake, respectively, is also located. Control devices that make it possible to drive both a derailleur in the two directions and a brake are commonly called integrated controls.
By convention, the device for controlling the front derailleur and the brake lever of the front wheel are located close to the left handgrip, and vice-versa the device for controlling the rear derailleur and the brake lever of the rear wheel are located close to the right handgrip.
EP 1 279 929 A2 discloses a transducer of angular quantities for a bicycle comprising a first and a second part which can perform a relative movement of rotation about a given axis, a magnetized element integrally fixed to one of said first and second part, and at least one pair of Hall effect sensors set angularly staggered with respect to one another about said given axis and integrally fixed to the other of said first and second part; said Hall effect sensors being sensitive to the presence of said magnetized element so as to generate respective output signals with values varying in a continuous range, the values of said respective output signals uniquely identifying the relative position of said first and second part with respect to said given axis. The values of the output signals also uniquely identify the direction of rotation of the first and of the second part with respect to the axis and the angular speed of rotation and/or acceleration. The transducer can be integrated in a motor/actuator, such as for example the motor/actuator of a motor-driven gearshift mounted on a cycle such as a competition bicycle.
Indeed, it is in general known to drive the derailleur of a bicycle gearshift with reference to values of a table of command values, each correlated to a position of the derailleur in which there is or takes place engagement of the chain with a specific toothed wheel. In order to drive the actuator, the control electronics uses a table of values containing, for each toothed wheel, the value that a variable of the derailleur must take up to position the chain in engagement with the toothed wheel. Such a value can be a differential value with respect to the adjacent toothed wheel, or it can be an absolute value with respect to a reference, for example with respect to a reference toothed wheel or to an end of stroke condition or a condition of lack of excitation of the motor.
From the point of view of magnitude, a command value of the actuator of the table of values can for example be the distance travelled by a mobile point taken as reference on the derailleur, the number of steps or revolutions that the motor should be made to perform, a length of excitation time of the motor, the value of a power supply voltage of a motor having an excursion proportional to the voltage, or furthermore it can be the value emitted by the sensor or transducer associated with the motor, a numerical value stored in a register and representative of one of the aforementioned amounts, etc.
In particular, the motors of the actuators can be driven for a number of steps or for a length of excitation time or with a voltage that are appropriate for each upward or downward gearshifting and then automatically stopped, while the sensors are used to provide a feedback signal to the control electronics so that it can possibly take care of actuating the actuator motors again in case the intended position has not been reached, namely the aforementioned variable of the derailleur, has not taken up the table value. This may for example be due to the fact that the resistant torque offered by the derailleur which to a certain extent depends on how the cyclist is pedalling, was too high, greater than the maximum torque that can be delivered by the motors through the linkage.
The values of said table of command values are nominal values, set in the factory, which take the number of toothed wheels in the derailleur (front or rear) and the respective thicknesses and distances between centres into account. Typically, such nominal values provide that, in the absence of the driving signal of the actuator, namely with command value at zero, the chain is in engagement with the toothed wheel having the smallest diameter, although as can be seen from the aforementioned examples, this condition is not necessary.
The table of values must accurately correspond to the electromechanical components of the gearshift, in particular to the distances between centres of the toothed wheels and/or to the mutual position of the elements of the motor or of the linkage taken as fixed reference and as mobile reference, as well as possibly to the progression of the actuation voltage of the motor, to the speed, acceleration and/or direction of rotation of the motor, etc.
Moreover, it is not unusual for the replacement of a derailleur to take place with a different model of derailleur, thus having different command values. The Applicant has realised that the actuation of a derailleur by reference to unsuitable command values can lead not only to temporary failure with possible worsening of performance, but also damage to the mechanics.
The problem at the basis of the invention is to provide an electronic bicycle system suitable for overcoming the aforementioned drawbacks.